Method for the production of an protective layer for a layer of luminescent material

ABSTRACT

A protective layer is provided for an image detector used for an x-ray image. This image detector comprises a layer of luminescent material that is to be protected against mechanical stress and moisture. A polymeric protective layer is disposed thereupon and is hardened exclusively in an area which does not border the layer of luminescent material. The hardened area provides protection against mechanical stress while the remaining area forms a moisture barrier. An appertaining method is provided for producing a polymeric protective layer on an image detector for an x-ray image, which is provided with a layer of luminescent material. The protective layer is deposited on the layer of luminescent material and then is hardened only in an area which does not border the layer of luminescent material.

BACKGROUND

Luminophore layers that operate as storage film (i.e., that store x-rayinformation) can be used for the generation of x-ray exposures. Suchstorage films are particularly used in digital radiography andmammography. The x-ray information is obtained by a process that beginswith the body to be examined being traversed by x-ray radiation. Afterthis irradiation, the x-ray radiation impinges on the storage film whereit effects changes on storage elements integrated into the storage film.The number of the storage elements thereby set depends on the intensityof the impinging x-ray radiation. Due to the spatial distribution of thestorage cells across the storage film, an x-ray exposure with the sizeof the exposed part of the storage film thereby results.

The storage elements of the storage film must be read out for generationof electrically-processable image data or image data visible to thehuman eye. The contents of the storage elements can be opticallyestablished. For readout, they are radiated with light of a specificwavelength and thereby optically excited. Such an excited storageelement emits light of a specific wavelength in the event that it wascharged or set beforehand via the absorption or x-ray radiation. Theintensity of the emission light thereby depends on the number of setstorage elements and therefore forms a measurement for thepreviously-absorbed x-ray radiation. The emission light is of arelatively lower intensity and is therefore measured withhigh-sensitivity detectors, for example, with photomultipliers.

The exposed storage film is read out pixel for pixel to generate anx-ray exposure. Electronic image data or image data perceivable by thehuman eye are generated from the read-out information. Due to theoptical readout of the storage film, very high requirements must beplaced on the uniformity of the film surface. Defects in the storagefilm affect not only the readout capability of the storage film, butalso the capability of engaging the storage cells via x-ray radiation.They reduce the achievable image quality in both events. The achievableimage quality therefore significantly depends on the freedom fromdefects.

Storage films are exposed to various mechanical stresses in x-raydiagnostic applications. For example, they are used in film cassettes inorder to generate diagnostic x-ray exposures in medicine. Film cassettesare used in “over-table” apparatuses in which the patient to be examinedis irradiation by x-ray radiation from above, with the patient lying onthe cassette and exerting a two-dimensional pressure on the cassette andtherewith on the storage film. The storage film is mechanicallystressed.

Moreover, contact with the patient leads to the creation of moisture onthe surface of the storage film, requiring the surface to be cleanedfrom time to time with a fluid-saturated cloth in order to removeadhering contaminants which likewise lead to the attachment of moisture.The quality of the storage film also suffers under the increase of thehumidity.

“Needle image plates” (NIP), in which the luminophore is grown on asubstrate in needle-shaped structures, are primarily used as storingluminophore layers. The needle tips of these structures end in thesurface of the storage film and influence the x-ray sensitivity andstorage capability of the film. Given support of the patient or subjectto be examined on a needle image plate, the needle ends situated in thesurface are mechanically stressed and can thereby be deformed. The x-raysensitivity and the storage capability suffer under the deformation.Needle image plates therefore require a particularly effectivemechanical surface protection.

From German patent document DE 100 48 810 A1, it is known to protect thesurface of needle image plates in that a deformable damping layer isapplied on the film surface. The damping layer thereby effects a uniformdistribution of mechanical loads and must, for its part, be protectedfrom scratches in order to not lose optical quality. For this purpose,it is proposed to apply a further cover layer made from SiO₂, Al₂O₃,TiO₂ or made from silicate. While the damping layer itself exhibits goodbonding properties with the needle image plate, upon application of thefurther cover layer, bonding problems occur with the cover layer thatcan only be remedied via extremely elaborate production methods—if atall. Should a parylene layer (poly-para-xylylene) be used as a dampinglayer due to its excellent properties, this fails to achieve asufficient bonding of the cover layer.

SUMMARY

The object of the invention is to specify a protective layer for aluminophore layer for x-ray exposures that offers excellent protection,both against mechanical loads and against humidity, exhibits a goodlayer bonding, and at the same time can be produced in an simple mannerand cost-effectively. A further object of the invention is to specify aproduction method for such a protective layer.

The invention achieves these objects via an apparatus with the featuresof the first patent claim and via a method with the features of thesixth patent claim. A basic idea of the invention is to provide apolymer protective layer that is hardened and in fact in a region thatdoes not abut the luminophore layer. By luminophore layer, what shouldthereby be understood are both storing and non-storing luminophorelayers. Polymer protective layers have the advantage that, for the mostpart, good bonding properties with luminophore layers can be achieved.Moreover, they can be produced in an uncomplicated and cost-effectivemanner. Furthermore, a sufficient resilience against mechanical loadsand against scratches is ensured by the hardness of the polymer.Likewise, uncomplicated and cost-effective methods are available forhardening such as, for example, electron beam hardening. Moreover, thepolymer most notably forms an effective barrier against moisture in thenon-hardened region. The only partially-hardened polymer protectivelayer therewith integrates protection against moisture and againstmechanical stresses and simultaneously ensures a layer design that canbe produced in a simple, durable and uncomplicated manner.

In an advantageous embodiment of the invention, the hardening of theregion of the protective layer not abutting the luminophore layer ensuesvia electron beam treatment. Electron beam treatment is can be realizedin a cost-effective and uncomplicated manner and moreover offers theadvantage that the parameters of the electron beam about to which depththe irradiated layer is treated (and therewith hardened) can be set veryexactly. The region of the protective layer that should not be hardenedcan thereby be set very exactly.

Further advantageous embodiments of the invention are the subject matterof the dependent patent claims.

DRAWINGS

Exemplary embodiments of the invention are subsequently explained usingFigures.

FIG. 1 is a pictorial diagram of a layer design according to anembodiment of the invention; and

FIG. 2 is a flowchart illustrating production method according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a layer design according to an embodiment of the invention.Shown is the protective layer 1 that lies over the luminophore layer 3.The luminophore layer 3 is applied on a substrate 5 on which it can beimprinted or vapor-deposited. It can be an arbitrary luminophore layer;in an embodiment of the invention, a needle image plate is used. Forexample, CsBr:Eu, RbBr:Tl or CsBr:Ga are used as storage luminophores,while CsI:Na or CsI:Ti are considered as non-storing luminophores, forexample. In particular, the storage luminophores that are preferablyused for needle image plates number among the alkali halogenides and cantake damage via moisture.

The material of the protective layer 1 is a polymer with suitablemechanical and moisture-resistant properties. A parylene layer ispreferably used that exhibits suitable protective properties and can behardened via temperature or electron beam treatment. The three parylenetypes N (poly-para-xylylene), C (chlorine-poly-para-xylylene), or D(di-chlorine-poly-para-xylylene) are particularly suitable for theelectron beam treatment. The thickness of the parylene layer is betweenapproximately 8 to 80 μm. Such layers can be imprinted, spun out (adistribution of the fluid parylene via centrifugal force due torotation), or vapor-deposited.

The protective layer 1 comprises a region 7 that does not abut on theluminophore layer 3 and a region 9 that abuts on the luminophore layer3. The non-abutting region 7 is hardened in order to form a surfaceresistant against mechanical stresses or scratches.

The hardening can be achieved in a simple manner by conventional methodssuch as temperature or electron beam treatment. However, the temperaturetreatment requires temperatures of at least 200-250° C. that would leadto re-crystallization of the luminophore layer 3 lying underneath.

Moreover, the temperature treatment exhibits the disadvantage that thelayer depth range in which it acts cannot be set well. This isdisadvantageous since the hardened region of the protective layer ismore permeable to moisture than the non-hardened region. The residual ofa non-hardened region of the protective layer 1 of a thickness of atleast 5 μm is therefore important to achieve the protective functionagainst moisture. Due to the better adjustability of the parameters, theregion 7 not abutting on the luminophore layer 3 is therefore preferablyhardened via electron beam treatment. The electron beam treatment allowsthe exact adjustment of the layer depth to be treated. The treatedregion 7 preferably exhibits a thickness of at least 3 μm in order toensure sufficient scratch protection of the surface.

Via the hardened region 7 and the non-hardened region 9, the protectivelayer 1 integrates protection against mechanical stress and scratchesand against moisture. At the same time, it can be applied with goodlayer bonding to the luminophore layer 3 underneath and represents aparticularly simple (because it is one piece) layer design.

FIG. 2 shows a manufacturer method according to the invention. It isthereby assumed that the luminophore layer 3 is already present on thesubstrate 5. Whether it is a storing or a non-storing luminophore layeris irrelevant.

The surface of the luminophore layer 3 is pre-treated in method step 11in order to offer good properties for the vapor deposition of theprotective layer 1. The pre-treatment ensues via what is known as plasmaetching in which the surface is fired upon with ions from a plasma. Thisplasma treatment, on the one hand, provides for a cleaning of thesurface at the atomic or molecular level; on the other hand, it effectsa micro-roughening of the surface that promotes a good layer bonding.

The polymer protective layer 1 is vapor-deposited in a subsequent methodstep 13. Pressure, spin, or evaporation methods are considered to bevapor deposition methods. A chemical vapor deposition method (CVD) ispreferably used. The CVD method can if necessary be physicallysupported, for example, via heat (a physically enhanced CVD, PECVDmethod). CVD methods ensure excellent layer bonding and layerproperties.

The protective layer 1 is treated by an electron beam in a subsequentmethod step. An electron beam of a specific energy is moved with aspecific speed over the surface of the protective layer 1. Theparameters of the electron beam and its movement over the protectivelayer influence the thickness of the region 7 of the protective layer 1that is treated. The electron beam treatment effects a hardening of theprotective layer 1 and increases its scratch resistance in a subsequentmethod step 15.

In a first example, a parylene layer of the type N with a totalthickness of 50 μm is treated. For this, an electron beam of 40 keV ismoved over the parylene layer via an electromagnetic x-y deflection. Theelectron beam speed is adjusted such that the uppermost 20 μm of thelayer are hardened. Since a plurality of further quantities influencethe depth of the treated region 7, the speed of the electron beam cannotbe exactly predetermined but rather must be determined experimentally.

In a second example, a parylene layer of the type C with a totalthickness of 30 μm is treated. For this, an electron beam of 25 keV ismoved over the parylene layer via an electromagnetic x-y deflection soquickly that the uppermost 5 μm are hardened.

In a third example, a parylene layer of the type D with a totalthickness of 20 μm is treated by an electron beam of 15 keV such thatthe uppermost 10 μm are hardened.

In a fourth example, a parylene layer of the type C with a totalthickness of 8 μm is treated by an electron beam of 5 keV such that theuppermost 3 μm are hardened.

In addition to an electromagnetic deflection of the electron beam, forexample, a mechanical feed of the layer can also be used for themovement of the electron beam relative to the protective layer.

For the purposes of promoting an understanding of the principles of theinvention, reference has been made to the preferred embodimentsillustrated in the drawings, and specific language has been used todescribe these embodiments. However, no limitation of the scope of theinvention is intended by this specific language, and the inventionshould be construed to encompass all embodiments that would normallyoccur to one of ordinary skill in the art.

The present invention may be described in terms of functional blockcomponents and various processing steps. Such functional blocks may berealized by any number of hardware and/or software components configuredto perform the specified functions. For example, the present inventionmay employ various integrated circuit components, e.g., memory elements,processing elements, logic elements, look-up tables, and the like, whichmay carry out a variety of functions under the control of one or moremicroprocessors or other control devices. Similarly, where the elementsof the present invention are implemented using software programming orsoftware elements the invention may be implemented with any programmingor scripting language such as C, C++, Java, assembler, or the like, withthe various algorithms being implemented with any combination of datastructures, objects, processes, routines or other programming elements.Furthermore, the present invention could employ any number ofconventional techniques for electronics configuration, signal processingand/or control, data processing and the like.

The particular implementations shown and described herein areillustrative examples of the invention and are not intended to otherwiselimit the scope of the invention in any way. For the sake of brevity,conventional electronics, control systems, software development andother functional aspects of the systems (and components of theindividual operating components of the systems) may not be described indetail. Furthermore, the connecting lines, or connectors shown in thevarious figures presented are intended to represent exemplary functionalrelationships and/or physical or logical couplings between the variouselements. It should be noted that many alternative or additionalfunctional relationships, physical connections or logical connectionsmay be present in a practical device. Moreover, no item or component isessential to the practice of the invention unless the element isspecifically described as “essential” or “critical”. Numerousmodifications and adaptations will be readily apparent to those skilledin this art without departing from the spirit and scope of the presentinvention.

1-8. (canceled)
 9. An image detector for an x-ray image, comprising: aluminophore layer; a protective layer lying over the luminophore layer,the protective layer being hardened only in a region not abutting theluminophore layer.
 10. The image detector according to claim 9, furthercomprising a non-hardened region that abuts the luminophore layer thatis at least 5 μm thick.
 11. The image detector according to claim 9,wherein the hardened region that does not abut the luminophore layer isat least 3 μm thick.
 12. The image detector according to claim 9,wherein the hardened region of the protective layer is anelectron-beam-treatment hardened region.
 13. The image detectoraccording to claim 9, wherein the protective layer is comprised ofpoly-para-xylilene.
 14. The image detector according to claim 9, whereinthe luminophore layer is a needle image plate.
 15. The image detectoraccording to claim 9, wherein the luminophore layer is comprised ofalkali halogenides or alkaline earth halogenides.
 16. The image detectoraccording to claim 15, wherein the luminophore layer is comprised ofCsBr:Eu, BaFBr:Ey, RbBr:Tl, CsBr:Ga, Csl:Na or Csl:Tl.
 17. A method forproducing a polymer protective layer on an image detector for an x-rayimage that comprises a luminophore layer, the method comprising:vapor-depositing the protective layer on the luminophore layer; andhardening only a region of the protective layer that does not abut theluminophore layer.
 18. The method according to claim 17, wherein aregion with a thickness of at least 5 μm that abuts the luminophorelayer is not hardened.
 19. The method according to claim 17, wherein aregion that does not abut the luminophore layer and that is hardened isat least 3 μm thick.
 20. The method according to claim 17, wherein thehardening ensues via electron beam treatment.
 21. The method accordingto claim 17, further comprising pre-treating the luminophore layer via aplasma treatment prior to the vapor-depositing of the protective layer.